Radio communication systems may have a cellular architecture, with each cell corresponding roughly to a geographical area. Each cell includes a base station (BS), which is a local central cite through which a number of radio transmitter/receiver units (user terminals (UTs)) gain access to the communications system. The UTs could be, for example, telephones, PDAs, or small modem boards. A UT establishes a communication link with other UTs by requesting access to the system through the BS. Each UT communicates over a communication channel distinguished from other UTs.
Various techniques exist to increase the number of available channels for a given number of available frequencies. Time division multiple access (TDMA), for example, divides a single frequency into multiple time slots. Each of the time slots can then be allocated to a separate communication channel. Other known techniques include code division multiple access (CDMA) and frequency division multiple access (FDMA), which, like TDMA, are considered conventional multiple access schemes.
Radio communications systems may employ a spatial division multiple access (SDMA) scheme, in conjunction with one or more conventional multiple access schemes, to increase the number of UTs that a BS can serve for a given number of available frequencies. An SDMA scheme may be implemented using a BS that has an array of receiver and/or transmitter antenna elements. The antenna elements are spaced, one from another, and may be positioned in various configurations. The array of antenna elements introduces a spatial dimension that can be used to differentiate two or more UTs concurrently accessing a given conventional channel. That is, the basis of an SDMA system is that the BS creates a spatially distinct SDMA channel for each of multiple users even though they share the same carrier frequency (FDMA), timeslot (TDMA), or spreading code (CDMA).
A significant communication problem can occur in which the assigned spatial channel of two UTs become interchanged. This phenomenon, known as channel swap, may occur for several reasons.
Channel swap may occur where the spatial processing capability of the SDMA system is exceeded. Spatial processing relies on the difference in geographical location between two UTs. It is possible, therefore, for two UTs (especially when mobile) to become physically located too close to one another for accurate differentiation using spatial processing, leading to an increased probability of channel swap.
Channel swap may also be the result of an unstable communication signal of one or more of the UTs. For example, channel swap may also occur when one or more SDMA channels experiences deep fading and communication with the BS is interrupted. Upon resuming communication, the communication link between each UT may be swapped.
Channel swap is not detectable by either the UTs or the BS. This is because only the data streams are from different UTs. The swapped signals appear identical in terms of physical characteristics such as bit structure, coding, and cyclic redundancy check (CRC), among others.
Channel swap can have serious detrimental consequences. Channel swap may cause the users at each UT to become involved in unintended communications. For systems that implement a secure communication scheme (e.g., a data stream encryption scheme), each UT will be employing an erroneous encryption key. The result will be that each user may receive only annoying “white” noise during a channel swap for such systems.